Evanescent field coupling into or out of an optical fiber has been done by placing a receiving core within the evanescent field of a sending core. In one approach, a fiber contaning two cores within a single cladding and substrate has been made. The fiber containing two cores was made by cutting a planar section from a preform and fusing two such cut preforms along the cut plane. The fused preform has two cores, and the cores in the fiber pulled from the cut preform transfer energy from one to the other because each lies in the evanescent field of the other. See Schiffner et al., "Double Core Single Mode Optical Fiber as Directional Coupler", Applied Physics, Vol. 23, p. 41, 1980.
Other approaches to obtaining evanescent field coupling include first pulling a fiber and then thinning the cladding by polishing or etching. A thin region of cladding then provides a path for the optical signal to couple into an adjacent medium by overlap of the evanescent field with the adjacent medium.
A problem which limits the usefulness of the technique of making a fiber containing two cores is the difficulty of coupling two separate single mode fibers to the double core fiber. The difficulty arises because the two cores are separated in the double core fiber by only a few microns.
A problem which limits the usefulness of the technique of first pulling the fiber and then thinning the cladding by polishing or etching is the difficulty of making a uniformly thin residual layer of cladding. In making an evanescent field coupler between two fibers, it is desirable to have uniformly thin regions of cladding on both fibers in order to achieve predictable coupling. The difficulty in polishing arises because optical fibers are small, on the order of 100 microns outside diameter and with a core which may be as small as 3 microns diameter, and polishing away cladding in order to leave a residual uniform thickness of a few times the core diameter is a severe constraint on polishing. Etching has been done by first twisting the fibers, then etching the twisted fibers, and finally encapsulating the etched fibers, and this approach makes fragile fibers with limited means for adjustment of the coupling.